Resiliency is the ability of a system to handle failures. A resilient system is a fault tolerant system.

Redundancy is duplication of component. It is one of the ways to provide resiliency.

Data stored on ZFS is resilient to memory corruption, bad disks and power outages.

The COW feature combined with RAID (with parity and hot spare) provided redundancy against memory and disk corruption. This in turn, makes ZFS data resilient.